1. Field of the Invention
The present invention relates to an apparatus, system and method for checking film for defects in a viewing angle widening film by using a simpler and easier process of optically checking the film for defects, and particularly for continuously checking the film in the process of manufacturing a viewing angle improving film for use in a liquid crystal display device or the like.
2. Description of the Related Art
Today, thin-film transistor (TFT) liquid crystal display devices and double supertwisted nematic liquid crystal (DSTN) display devices are being widely used as liquid crystal display devices. These liquid crystal display devices have a viewable region dependent on the viewing angle, and the display screen cannot be suitably viewed along a line out of the viewable region. For example, when the viewing angle is inclined up and down in a vertical direction, the color of the entire display screen becomes lighter and the contrast is lowered, or gray-scale inversion of a black display portion occurs to considerably reduce the viewability. In the case of large-screen liquid crystal display devices, such a contrast reduction or gray-scale inversion occurs more easily since the viewing angle is widened along with the increase in the area of the display screen. Therefore, there is a demand for a liquid crystal display device having a wide viewable region.
Under these circumstances, various means exist for improving viewing angle characteristics of liquid crystal display devices, e.g., a method of performing divided liquid crystal orientation in a liquid crystal display device, and a phase-difference film used as an optical compensation film that has a negative birefringence index have been studied.
For example, Japanese Patent Application Laid-open No. 6-214116 of the invention filed by the applicant of the present invention discloses an optical anisotropic element and a method of manufacturing the optical anisotropic element. According to the description in the publication, since liquid crystal molecules in liquid crystal cells of a liquid crystal display device are slightly tilted relative to the substrate of the liquid crystal display device when a voltage is applied, the liquid crystal display device can be regarded as a positive uniaxial optical anisotropic element having an optical axis in a direction slightly tilted relative to the substrate of the liquid crystal display device. Accordingly, it is possible to obtain a favorable liquid crystal display device having no viewing angle dependence by slightly tilting the optical axis of a negative uniaxial optical anisotropic element according to the above-mentioned tilt and compensating the phase difference caused by the liquid crystal cells with a phase difference caused by the optical anisotropic element. Based on this principle, the Applicant of the present invention manufactures a film which forms the optical anisotropic element as a wide view film for improving the angle at which a liquid crystal display device may be viewed.
To uniformly maintain liquid crystal cells in an optimized compensated state, a high degree of uniformity is required of the above-described liquid crystal viewing angle improving film constituted of a low-molecular liquid crystal and forming an optical anisotropic element. However, the above-described liquid crystal viewing angle improving film is manufactured through various complicated steps, including application of a liquid crystal to a flexible supporting member, drying, orientation and hardening of film. Therefore, there is a possibility of mixing and adhesion of foreign materials in and to the liquid crystal material to variously or randomly disorder the orientation of molecules of the low-molecular liquid crystal, and there is also a possibility of application non-uniformity causing variation in retardation value, resulting in generation of various defective portions which do not have the desired optical characteristics.
It is desirable that all of such defective portions be detected with accuracy in a manufacturing line for obtaining the above-described liquid crystal viewing angle improving film through a complicated manufacturing process, and that no liquid crystal viewing angle improving film having such defective portions be put on the market.
As a method for detecting defects in liquid crystal viewing angle improving films in a state of being conveyed in a manufacturing line, a method has been used in which the liquid crystal viewing angle improving film is interposed between a pair of polarizers one of which has, on one side of the liquid crystal viewing angle improving film, a polarizing transmission axis parallel to the direction of conveyance of the film, and the other of which has, on the other side, a polarizing transmission axis perpendicular to the direction of conveyance of the film, i.e., a pair of polarizers placed as crossed Nicols; detecting illumination light is projected from the outside of one of the pair of polarizers to the liquid crystal viewing angle improving film; transmitted light emergent from the opposite side is received with a line sensor or the like to obtain a signal representing the luminance of the transmitted light; and the luminance signal is processed by, for example, mathematical differentiation to detect a defective portion in the film surface according to a change in the luminance signal.
This method, however, entails a problem described below. When a luminance signal obtained by imaging with a charge-coupled device (CCD) camera or the like is displayed so as to form an image, a defective portion forms a brighter image portion in contrast with the background normal portion. However, the quantity of light transmitted through the normal portion is nonuniform because of the viewing angle dependence of the liquid crystal viewing angle improving film itself. Therefore, the signal-to-noise ratio of the luminance signal of the brighter image portion corresponding to the defective portion is low and the defective detection accuracy is low. Further, a nonuniform background portion in the normal portion may be removed by shading correction calculation. However, since the information in the luminance signal is also processed by the calculation, this method is not effective in improving the detection accuracy.
The above-described detection method makes it comparatively easy to detect defective portions by placing as crossed Nicols a pair of polarizers on the opposite sides of the liquid crystal viewing angle improving film with one of their polarizing transmission axes set parallel to the direction of conveyance of the film. Since the orientation of the liquid crystal in the liquid crystal viewing angle improving film is generally perpendicular to the polarizing transmission axis, the quantity of light transmitted through the surface of the normal film is small. However, the quantity of light transmitted through defective portions that form a brighter image region is also generally small, so that the signal-to-noise ratio of the luminance signal corresponding to defective portions is low and the defective detection accuracy is low.
Also, in the above-described method, a check of the film for defects in a liquid crystal viewing angle improving film in a state of being conveyed is made by using a line sensor having solid-state image pickup elements arranged in a row in a direction perpendicular to the direction of conveyance. Defects which appear continuously or periodically parallel to the arrangement of the solid-state image pickup elements, e.g., a stepped nonuniformity in a direction of the film thickness resulting from application nonuniformity, cannot be accurately detected from a change in the luminance signal obtained by the above-described line sensor.
Accordingly, it is not possible to accurately and reliably detect all defects caused during the manufacturing process.
This is not only a problem of the liquid crystal viewing angle improving film but also a problem common to all phase-different films utilizing any type of birefringence index.
In view of the above-described problem, an object of the present invention is to provide an apparatus, system and method for checking a film for defects in a manufacturing line or the like by using a simpler and easier process of optically detecting defects in the film, such that all optical defects caused during the manufacturing process can be accurately detected without any fail to be detected, and particularly to provide a film defect detecting system for making a continuous check of a film for defects due to mixing of a foreign material, orientation nonuniformity, stepped nonuniformity, etc., in the process of manufacturing a viewing angle improving film for use in a liquid crystal display or the like to accurately detect all of such defects without any fail.
To achieve the above-described object, according to one aspect of the present invention, there is provided an apparatus for checking a film for optical defects, comprising a pair of polarizers placed on opposite sides of a film surface of a film to be checked with respect to optical defects, the polarizers being parallel to the film, an illumination light source placed outside the pair of polarizers, the illumination light source projecting light onto the film through: a first polarizer of the pair of polarizers, light receiving unit placed outside the pair of polarizers and opposite from a placed position of the illumination light source, the light receiving unit receiving light transmitted through a second polarizer of the polarizers, and a correction film having substantially the same birefringent characteristics as a portion of the film having no optical defects, a direction in which the correction film is to be positioned being previously set according to the birefringent characteristics of the film, the correction film being placed in one of two spaces between the pair of polarizers and the film parallel to the film, characterized in that the luminance signal of light transmitted through the film, the correction film and the pair of polarizers is obtained by a light receiving unit to check the film for optical defects.
In the above-described apparatus, a portion of the film having no optical defects may be placed as the correction film after being rotated through 180 degrees along a plane corresponding to the film surface or after being reversed from front side to back side. Also, the correction film may be adhered to one of the pair of polarizers.
Preferably, the apparatus has an optical system for converging transmitted light on the light receiving unit, the optical system being provided in an optical path for the transmitted light between the light receiving unit and the second polarizer.
The light receiving unit may include solid-state image pickup elements, or other well-known image pickup elements. When the film is checked for optical defect during conveyance of the film, the light receiving unit comprises a plurality of solid-state image pickup elements arranged in a row inclined relative to a direction perpendicular to a direction of conveyance of the film.
Preferably, the pair of polarizers are placed in crossed-Nicols relationship with each other.
According to another aspect of the present invention, there is provided a film defect check system for checking a film continuously conveyed for optical defect, the system comprising a plurality of film defect checking apparatuses placed in a path through which the film is conveyed, each of the film defect checking apparatus having a pair of polarizers placed on opposite sides of a film surface of the film to be checked with respect to optical defects, the polarizers being parallel to the film, an illumination light source placed outside the pair of polarizers, the illumination light source projecting light onto the film through a first polarizers of the pair of polarizers, light receiving unit placed outside the pair of polarizers and opposite from a placed position of the illumination light source, the light receiving unit receiving light transmitted through a second polarizer of the polarizers, and a correction film having substantially the same birefringent characteristics as a portion of the film having no optical defects, a direction in which the correction film is to be positioned being previously set according to the birefringent characteristics of the film, the correction film being placed in one of two spaces between the pair of polarizers and the film parallel to the film, characterized in that directions of polarizing transmission axes of the pairs of polarizers of the plurality of film defect checking apparatuses are respectively set at different angles from a direction of conveyance of the film continuously conveyed.
Preferably, in the above-described system, a number of the film defect checking apparatuses is at least three, and the pairs of polarizers of the at least three film defect checking apparatuses are placed so that the angles each of which is an angle of intersection of one of the pair of polarizers in each of film defect checking apparatuses and a phase-delay axis of a portion of the film having no optical defects include an angle of about zero, an angle in a range of 5 to 15 degrees, and an angle in a range of 35 to 45 degrees.
Preferably, in at least one of the film defect checking apparatuses, the light receiving unit comprises a plurality of solid-state image pickup elements arranged in a row inclined relative to a direction perpendicular to the direction of conveyance of the film.
Preferably, the pair of polarizers are placed in crossed-Nicols relationship with each other.
According to still another aspect of the present invention, there is provided a method of checking a film for defects, in which a film interposed between a pair of polarizers parallel to each of the pair of~polarizers is checked for optical defects by projecting illumination light from the outside of a first polarizer of the pair of polarizers, and by receiving light transmitted through a second polarizer of the polarizers, the method comprising the steps of placing a correction film which has substantially the same birefringent characteristics as a portion of the film having no optical defects in one of two spaces between the pair of polarizers and the film parallel to the film surface of the film, and a direction in which the correction film is to be positioned being previously set according to the birefringent characteristics of the film, and projecting the illumination light to the first polarizer.
directions of polarizing transmission axes of the pair of polarizers are set according to respective types of defects in the film, the pair of polarizers keeping crossed-Nicols relationship with each other.